Methods of scrambling composite television signals in a manner to allow subsequent unscrambling have been proposed and a number of these methods are in use. A number of these prior art systems are exemplified in U.S. Pat. No. 3,106,604, U.S. Pat. No. 3,538,243, U.S. Pat. No. 3,147,061 and U.S. Pat. No. 2,619,530. The most basic of the scrambling techniques have used what is referred to as synchronization suppression or synchronization inversion which basically alters the horizontal synchronization pulse in a manner so that a television set will not recognize it thereby resulting in the signal being unacceptable for reproduction on the television. Although this technique works quite well, a person wishing to pirate the signal can easily identify the scrambling technique and produce an apparatus which will correct the signal such that it is acceptable for the television. Because of this, more complicated scrambling techniques have been proposed which would include line inversion and line shuffling.
Line inversion is a fairly good technique to stop a would-be pirate in that a line of video information can be inverted about a particular axis and it is more difficult for the pirate to reverse this process as he must recognize what lines have been inverted and about what axis they have been inverted. Although this techniqueworks satisfactorily on a lab bench it has proven difficult to maintain the quality of the signal in practise. This is primarily due to the nature of communication channels used by cable companies which introduce noise during transmission adversely effecting the signal decreasing the precision possible upon unscrambling of the signal.
Line shuffling is another known more sophisticated technique which stores complete lines of video information including all the synchronization information, to alter the transmission sequence of lines of video information. The unscrambler then reverses this operation as it has knowledge of the method of scrambling. In order to be effective many lines should be scrambled and the separation or the shuffling should be a minimum of about 6 lines. This increases the cost of the unscrambling unit as it must be capable of storing lines equal to the maximum separation.
Other methods of scrambling television signals have been proposed altering the time relation between horizontal synchronization information and video information.
Systems of this type generally only cause limited horizontal shearing. Systems of the type mentioned above as well as other scrambling systems and devices are disclosed in U.S. Pat. Nos. 3,244,806, 3,732,355, 2,972,009 and 2,758,153.
There still remains a need to provide a scrambling method which is sufficiently sophisticated to prevent would-be pirates while providing a method which does not result in the degradation of the signal due to transmission through the known communication channels. In addition the scrambling technique must be simple enough to allow a low cost reliable unscrambler unit as for any given scrambling system there will be a very large number of unscrambling units which are used by subscribers.